Hereinafter, a device-to-device communication environment of the present invention will be described.
Examples of a device-to-device communication method include a device-to-device (D2D) communication method, a machine-to-machine (M2M) communication method and a peer-to-peer (P2P) communication method. D2D communication means communication between electronic devices. In a broad sense, D2D communication means wired or wireless communication between electronic devices or communication between a machine and a device controlled by a person. However, recently, D2D communication generally means wireless communication between electronic devices performed without human intervention.
In the early 1990's, the concept of device-to-device communication was only recognized as the concept of remote control or telematics and the market therefor was limited. In the past few years, D2D communication has rapidly grown into a market globally attracting considerable attention. In particular, device-to-device communication applies leverage to fields such as fleet management in point of sale (POS) and security application markets, remote monitoring of machines and equipment, and smart meters for measuring operation times of construction equipment and automatically measuring heat or electricity consumption. In the future, device-to-device communication will be variously used in association with existing mobile communication and wireless high-speed Internet or low-power communication solutions such as Wi-Fi and ZigBee and will expand to a Business-to-Consumer (B2C) market as well as a Business-to-Business (B2B) market.
For ease of description, hereinafter, device-to-device communication is referred to as D2D communication. A terminal used for device-to-device communication in a device-to-device communication environment is referred to as a D2D device. Device-to-device communication may be used not only in a communication environment in which only devices exist but also in an environment in which devices and general cellular terminals (e.g., human type communication (HTC) terminals) coexist.
In device-to-device communication, in an environment in which devices and general HTC terminals coexist, D2D devices may perform transmission and reception of data and/or messages using resources allocated by a base station under control of the base station. In this case, the D2D devices should perform synchronization with the base station in advance in order to acquire reference timing and/or basic timing slot information.
At this time, when a D2D device is connected to a cellular network using existing infrastructure, for connection and synchronization with the network, preamble detection may be performed in downlink and connection with a base station and time-frequency synchronization may be performed via transmission of a ranging signal and/or a ranging channel to the base station in uplink.
Even after pre-synchronization with the base station, in order to perform direct communication between D2D devices, synchronization between devices must be necessarily performed for accurate decoding/detection at a message/data level. When only communication between devices close to each other, the locations of which are fixed, is considered, D2D communication may be supported by synchronization with the base station only within a given cyclic prefix (CP). However, in order to support a D2D service of wider coverage and a mobile terminal, synchronization between devices should be considered.
However, for data transmission and reception under accurate synchronization between devices, when an additional synchronization signal is exchanged separately from synchronization with the base station in cellular communication, overhead and latency may occur in the D2D communication system.
For example, in general, a random access channel (RACH) signal transmitted from one terminal to a base station is transmitted to a serving base station (or a neighbor target base station upon handover). Through transmission of this RACH signal and a feedback message (i.e., RACH response message) of the base station, the terminal may simultaneously perform power adjustment and identification of the terminal accessing the network after uplink time-frequency synchronization.
However, in an existing method, D2D devices should perform a random access procedure separately from the base station and a process of performing synchronization between D2D devices must be necessarily performed. Therefore, complexity of the D2D device may be increased and power consumption of the D2D device may be increased.